Conventionally known magnets containing rare-earth elements (hereinafter referred to rare-earth magnets) are generally brittle and inferior in strength against tension, bending, torsion and the like and in machinability. Furthermore, there is a problem that it is difficult to increase bonding strength between these rare-earth magnets and metals of rotor member such as electric motor and the like. For example, an example of a schematic structure of a rotor used in a permanent magnet type high-speed electric generator, motor or the like, in which a rare-earth magnet is used, will be explained, by reference to FIGS. 46a and 46b. In these drawings, 1, 2, 3 and 4, 4 are a shaft, a rare-earth magnet, a metal cylinder of non-magnetic material, and a metal disk of non-magnetic material, respectively.
The above rare-earth magnet 2 is a magnet of high magnetic energy product, containing as a main component an active rare-earth element, such as an Nd--Fe--B magnet, in which neodymium is used, a Pr--Fe--B magnet, in which praseodymium is used, and a Sm--Co magnet, in which samarium is used. This rare-earth magnet 2 tends to corrode. Therefore, it is provided with an epoxy coating, an aluminum-chromate film or a nickel plating with copper bed. In its bonding with a rotor member metal of a metal cylinder 3 or the like, a means for bonding using a polymer bond such epoxy resin is taken.
In case of the above-mentioned rotor structure, the rare-earth magnet 2 is low in mechanical strength, as mentioned above. Therefore, the rare-earth magnet 2 is inserted into the non-magnetic metal cylinder 3, and non-magnetic metal disks 4 are shrunken on both ends of the metal cylinder 3 in order to hold both end portions of the metal cylinder 3. With this, the rare-earth magnet 2, which is disposed in the inside of the metal cylinder 3, is held by the metal cylinder 3 and the metal disks 4. Thus, the rotor is maintained in strength and rigidity.
Furthermore, according to a conventional example of FIGS. 47a and 47b, an iron core 5 is disposed about the shaft 1, a plurality of iron core grooves 6, which have a wedge-shaped section, are formed on the peripheral portion of this iron core 5 along the longitudinal direction, rare-earth magnets are disposed in the inside of the iron core grooves 6, and a means for bonding them by using a polymer bond such as epoxy resin is taken. Furthermore, the actual state is that the rare-earth magnets 2 are held by using a reinforcement by hoops using a plastic material (FRP) reinforced with aramid or glass fibers, together with the metal cylinder 3, and thus a rotor resistant to high speed rotation is realized.
The magnetic characteristics are improved remarkably by the appearance of the rare-earth magnet. Permanent magnet type synchronous machines, where these powerful magnets are built in their rotors, have an energy density per unit area that is higher than that of induction machines or coil-type synchronous machines. Therefore, it becomes possible to improve the output, if the rotation speed can be increased. Furthermore, there is an advantage that it becomes possible to make an electric motor or generator smaller in size and improve the same in performance. However, the following problems exist.
First of all, rare-earth magnets made by powder sintering method are inherently brittle materials, and insufficient in mechanical characteristics such as strength, tenacity, toughness and deformative capability, as compared with iron core and other metal materials constituting a rotor. Therefore, if centrifugal force, which acts on a rotor, increases further by the speedup or increase of capacity of electric machines and the like, there arises a problem that a rare-earth magnet tends to deform or break.
For example, an Nd--Fe--B magnet is about 260 (MPa) in flexural strength, and this is not larger than a half of that of a common steel. Its elastic modulus is about 150 (Gpa), and this is about 3/4 of that of steel. Furthermore, its breaking elongation is about 0.2%, and this is not larger than 1/10 of that of steel and is very small. Furthermore, it comes to break by only elastic deformation with little plastic deformation. However, it has a characteristic that its compressive strength is two or more times flexural and tensile strengths. A Pr--Fe--B magnet has also a strength that is almost equal to this, but strength of a Sm--Co magnet is still small.
Furthermore, the presence of internal defects, such as voids and small cracks, contained in a magnet prepared by a powder sintering in an argon gas atmosphere under a normal or somewhat vacuum pressure is considered to be one of the causes that make powder sintering magnets lower in strength.
Secondly, it can be said that rare-earth magnets are insufficient in corrosion resistance. In each of Nd--Fe--B magnets, Pr--Fe--B magnets and Sm--Co magnets, each of rare-earth elements of neodymium, praseodymium and samarium is active. Thus, if these rare-earth magnets are allowed to stand still for several days in the atmosphere, their surface will have a different color and their corrosion will proceed. Therefore, they are put into a practical use in general under a condition that they are provided with an epoxy coating, an aluminum-chromate film or a nickel plating with copper bed.
Thirdly, it can be said that bonding strength between a rare-earth magnet and a rotor member metal is insufficient. For example, tensile strength between a rare-earth magnet and copper, which is one of rotor member metals, that have been bonded together with an epoxy resin bond is about 20 (MPa) at room temperature, and this is about 1/4 of tensile strength of an Nd--Fe--B magnet.
Furthermore, the bonding strength lowers further at a high temperature that is higher than 100.degree. C. Therefore, bonding strength can hardly be expected in a rotor portion that generates heat to have a temperature of at least 100.degree. C. when it is driven. Furthermore, the heat resisting temperature of a common Nd--Fe--B magnet under use is a maximum of 140-160.degree. C.
Fourthly, it can be said that high-strength bonding technique, which does not deteriorate magnetic characteristics of rare-earth magnets, has not yet been established. That is, it is the actual condition that high-strength bonding technique between magnet and rotor member metal, which can endure a temperature higher than 100.degree. C. caused by heat generated by a rotor, without deteriorating magnetic characteristics of rare-earth magnets in themselves, has not yet been established. As stated above, rare-earth elements are extremely active. Therefore, even though someone tries to braze a rare-earth magnet and a rotor member metal, with a brazing filler metal such as silver, under a high temperature of about 850-900.degree. C., the rare-earth element and the brazing filler metal react violently. Thus, it is extremely difficult to bond them without deteriorating magnetic characteristics of the magnets. Furthermore, the bonding strength becomes not larger than 10 (MPa). There is a problem that silver element as a brazing filer metal diffuses deep into the inside of Nd--Fe--B magnet, and thus coercivity of the magnet lowers greatly.
In connection with the above-mentioned problems, as a means for increasing bonding strength between a rare-earth magnet and a rotor member metal, a method for bonding a rare-earth magnet and another member, such as carbon steel, silicon steel plate, or flat-rolled steel plate, which is different from the magnet, is proposed, for example, in Japanese Patent First Publication JP-A-8-116633. In this method, a bonding member made of a rare-earth based alloy (Nd--Cu alloy) is interposed between the rare-earth magnet and the another member, and they are heated to at least a temperature, at which the liquid phase of the bonding member occurs, and thus bonded using wettability by liquefaction of the bonding member.
JP-A-7-116866 proposes a method for conducting a diffusion bonding between a rare-earth magnet's raw material, which has not yet been magnetized, and a supporting member such as carbon steel or stainless steel, by a hot process where they are pressurized under a particular condition.
Although its object is not a rare-earth magnet, JP-A-7-232284 discloses a method for conducing a diffusion bonding between a titanium alloy member and an iron based metal member, with an intermediate member that is a combination of a vanadium thin plate (or tantalum thin plate) and a copper thin plate. In this method, the titanium alloy member, the vanadium thin plate, the copper thin plate, and the iron based metal member are disposed in sequence and are bonded together by diffusion by holding them under pressure after heating them at a temperature lower than the melting point of copper.
Furthermore, JP-A-1-171215 (Japanese Patent No. 2571244) proposes a method for producing a rare-earth-Fe--B metal laminate. In this method, a magnet having a basic composition that is a combination of a rare-earth element (Y is included therein), a transition metal, and boron is produced. This magnet and another object for forming the laminate are put into a hermetic container, and then the container is sealed in a vacuum. Then, they are subjected to a hot isostatic pressing treatment at a temperature of 850-1,000.degree. C. With this, the magnet and the another object are monolithically bonded together. An iron core, iron plate or ceramic is used as the another object.
However, according to the above proposal of JP-A-8-116633, the rare-earth based alloy as the bonding member is high in activity. Therefore, it is easily oxidized by a reaction with oxygen, and there arise problems that the resultant oxide comes off the bonded portion, thereby to lower the bonding strength, and surrounding devices are contaminated. Furthermore, the bonding member in itself contains a large amount of rare-earth element of high price. Thus, there is a problem that the raw material cost increases.
According to the proposal of JP-A-7-116866, the pressurization by the hot process is a uniaxial pressurization. Therefore, in case that the surfaces to be bonded together are curved surfaces, it is difficult to uniformly pressurize the entire surfaces. Thus, the bonding strength varies. Furthermore, for example, in case that a supporting member is bonded to the entire peripheral surface of a cylindrical magnet, this method can not be adopted. Thus, there is a problem that the shape of an object to be bonded is limited.
The proposal of JP-A-7-232284 is a method for bonding a titanium alloy member with an iron based member, which is a so-called structural material, such as steel and stainless steel. It is impossible to directly apply them to rare-earth magnets that are completely different therefrom in texture and condition.
According to the proposal of JP-A-1-171215, the rare-earth magnet is a cast magnet, and the another object for forming the laminate is an iron core, an iron plate, a ceramic, and the like. Thus, it is different from an alloy material of the invention, which is a high melting-point metal or high specific-tenacity material, as an object to be laminated.
Thus, the present invention was made in view of the above, and its object is to provide a high-strength bonding structure and a bonding method that are capable of monolithically bonding a rare-earth magnet with an alloy material that is a high melting-point metal or high specific-tenacity material, without deteriorating magnetic characteristics, such that the rare-earth magnet's insufficiency in strength, rigidity, tenacity and the like is compensated.